This invention relates to a control device for a direct injection engine in which fuel is injected directly into a combustion chamber in a cylinder by means of a fuel injection valve, and such an injection engine. More particularly, the invention is concerned with a technique of throttle control during acceleration of a direct injection engine which can be switched between uniform charge combustion mode and stratified charge combustion mode.
A known example of a conventional control device for this type of direct injection engine is found in Japanese Unexamined Patent Publication No. 7-301139, in which the engine is operated in stratified charge combustion mode by injecting the fuel into a cylinder during a compression stroke in a specific low-speed, low-load operating range of the engine while it is operated in uniform charge combustion mode by injecting the fuel into the cylinder during an intake stroke in other operating ranges. More specifically, a region of a relatively rich mixture is formed close to a spark plug and a stratiform mixture is formed around the region of the rich mixture in the stratified charge combustion mode. Since the engine is run at an extremely high average air-fuel ratio within a combustion chamber in the stratified charge combustion mode, it is possible to significantly increase the opening of a throttle valve compared to its opening in the uniform charge combustion mode, and this helps decrease pumping loss of the engine and achieve a reduction in fuel cost.
The aforementioned control device for the direct injection engine however has a problem that torque shock occurs when the engine is switched from the stratified charge combustion mode to the uniform charge combustion mode during acceleration, for instance. Since the throttle opening in the stratified charge combustion mode is larger than the throttle opening in the uniform charge combustion mode as stated above, the throttle valve is usually closed for an instant when the combustion mode is switched during engine acceleration. Even if the throttle valve is temporarily closed in this fashion, there occurs a considerable time delay before the charging efficiency in the cylinder actually drops and, therefore, the charging efficiency in the cylinder increases up to an excessive level, thereby increasing engine torque excessively when the engine is switched to the uniform charge combustion mode.
One approach to reducing such variations in engine torque would be to control the throttle valve in such a way that its opening becomes as small as possible in the stratified charge combustion mode. Even when this approach is used, however, the opening of the throttle valve is increased in accordance with an increase in target load level during acceleration of the engine, so that the throttle opening is fairly large immediately before the engine is switched to the uniform charge combustion mode and the aforementioned torque shock inevitably occurs after all at the time of combustion mode switching.
Another approach would be to delay the switching to a fuel injection pattern of the uniform charge combustion mode until the amount of intake air decreases as disclosed in Japanese Unexamined Patent Publication No. 4-36221, or to temporarily close the throttle valve as described above and delay the switching of the fuel injection pattern until the charging efficiency in the cylinder actually decreases, when switching the engine to the uniform charge combustion mode. Even when such an approach is taken, however, there arises a problem that vehicle acceleration performance deteriorates since transition to the uniform charge combustion mode is practically retarded, also causing a delay in increasing the amount of fuel.
Still another approach would be to forcibly decrease the engine torque by retarding ignition timing by a specific time period to alleviate the torque shock when switching the combustion mode. However, this approach makes it necessary to significantly retard the ignition timing so that there arises a problem that fuel economy considerably deteriorates. Furthermore, the reliability of a catalyst for converting exhaust gas might be impaired due to an excessive increase in exhaust gas temperature.